Busbar thickness and width: what drives the choice

Why area alone does not settle the section

The first step in busbar sizing is usually the cross-sectional area. Current, allowable temperature rise and voltage drop give a minimum area, and a standard size is chosen above it. That is necessary, but it is not the whole decision. The same area can be arranged as a wide, thin bar or a narrow, thick one, and the two behave differently in service.

Current, area and temperature rise

For a given material, the resistance of the bar falls as the area rises. The heat generated per unit length is the current squared times the resistance per unit length, so doubling the area roughly halves the heat per unit length. The temperature rise depends on how that heat leaves the bar, which is where the shape starts to matter.

A busbar in an enclosure loses heat by convection and radiation from its exposed surface, and by conduction into its supports and joints. The surface area per unit length is the perimeter of the section. A wide, thin bar has a larger perimeter than a narrow, thick bar of the same area, so it has more surface to lose heat from.

That does not mean wider is always cooler. A wide bar mounted flat against an insulating panel loses the benefit of one face, and a bar in a tight stack may see little air movement. The installation decides how much of the perimeter is useful. The calculation should use the mounting condition, not the free-air case.

Skin effect and frequency

At mains frequency, the current in a copper busbar is spread fairly evenly through the section. The skin depth in copper at 50 Hz is around 9 mm, which is comparable to the thickness of heavy bar but not small compared with it. For most low-voltage busbars, the effect is modest and the direct current resistance is a reasonable starting point.

At higher frequencies the current crowds towards the surface, and the effective resistance rises. A thin, wide bar can then be better than a thick one because it presents more surface for the same area and avoids carrying current in a centre that the current does not use. Laminated or stranded bars are used where the frequency is high enough for this to matter.

Bending stiffness and the bend direction

Bending stiffness depends on the second moment of area, which grows with the cube of the dimension in the direction of bending. For a bar loaded flatwise, the thickness is the small dimension and the stiffness is proportional to width times thickness cubed. For a bar loaded edgewise, it is proportional to thickness times width cubed.

That is why a thin, wide bar is flexible in the flatwise direction and stiff in the edgewise direction. The same bar can be easy to route around a corner and hard to deflect sideways. A thick, narrow bar behaves the other way. If the bar has to resist vibration or hold a component, the direction of the load decides which section works.

Flatwise and edgewise bends also behave differently in the tool. An edgewise bend puts the neutral axis through the width, and the outer fibre sees more strain for the same radius. If the design needs an edgewise bend at a tight radius, check the thickness and temper before releasing the drawing.

Space, joints and hole patterns

The available space often decides the section before the calculation does. A bar that is 100 mm wide may not fit between a breaker and the enclosure wall, while a 50 mm by 10 mm bar of the same area will. The reverse can happen where the stack height is limited and a thin bar is the only option.

Hole patterns constrain the width. A row of holes for a terminal needs enough edge distance, and a wide bar gives more room for a spread pattern. A narrow bar may force a single row, which concentrates the joint and may reduce the contact area.

The joint face is easier to keep flat on a thicker bar, because the section resists bending and the bolt load is spread over a stiffer plate. A thin, wide bar can bow between bolts, which opens the joint at the edges. If the joint is critical, thickness is often the more useful dimension to increase.

Clearance and creepage to other phases also depend on the shape. A wide bar may reduce the air gap to a neighbouring part, while a thick bar may reduce the creepage path over a support. Check the insulation envelope after the section is chosen, not before.

Standard sizes, cost and availability

Busbar is stocked in a range of standard widths and thicknesses, and the price per kilogram does not vary much with size. What varies is the availability and the scrap. A non-standard width may need a mill order or a cut from a wider bar, which adds cost and lead time.

The metal cost follows the mass, so the section choice should not change the metal bill much for the same area. The conversion cost does change. A very thick bar needs a heavier press or a different shear, and a very wide bar needs a wider tool and more careful handling. A thin bar may need more supports and more careful packing to avoid damage.

A selection sequence that works

Start with the current and the allowable temperature rise, using the enclosure condition rather than free air. Get a minimum area. Then look at the space: the available width, the available thickness and the clearance to other parts. That usually narrows the choice to two or three standard sizes.

Check the short-circuit withstand for the clearing time. A section that is adequate for the continuous current may not survive the fault, and the support spacing affects the electromagnetic force the bar has to resist. The fault case can set the thickness even when the continuous case does not.

Check the bend route. If the bar has to bend flatwise around a corner, a thinner section helps. If it has to resist vibration or hold a joint flat, a thicker section helps. If both are needed, consider a different support arrangement or a fabricated assembly rather than forcing one bar to do everything.

Finally, check the stock size and the joint layout. Adjust the supports, the hole pattern or the phase spacing to use a standard bar where possible. Then confirm the choice with a temperature rise test or a calculation that matches the installation, and record the assumptions with the drawing.

• Minimum area from current and allowable rise in the real enclosure
• Short-circuit withstand for the fault level and clearing time
• Available width, thickness and clearance to other parts
• Bend direction, minimum radius and support spacing
• Joint face flatness and hole pattern with edge distance
• Standard stock size and mill tolerance on the section